(a) Field of the Invention
The present invention relates to a liquid crystal alignment agent and a liquid crystal alignment film formed therefore, as well as a liquid crystal display element provided with the liquid crystal alignment film; and more particularly provides a liquid crystal alignment agent and a liquid crystal alignment film formed therefore, as well as a liquid crystal display element provided with the liquid crystal alignment film without image sticking problems and having superior voltage holding ratio.
(b) Description of the Prior Art
At present, the polymers such as polyamide acid, polyimide, and the like, are used as a liquid crystal alignment agent, after coating onto a substrate having a transparent conducting film, heating and alignment process to form a liquid crystal alignment film for the liquid crystal display element. Finally, two of the substrates coated with alignment film are placed in opposite directions to form a cell gap holding a liquid crystal layer between the two substrates.
Nematic liquid crystal display elements are predominantly used in general liquid crystal display elements, and examples of types of nematic liquid crystal display elements actually used include: (1) a TN (Twisted Nematic) liquid crystal display element, comprising a liquid crystal alignment direction of one side substrate placed at a 90 degree angle to a liquid crystal alignment direction of the other side substrate; (2) a STN (Super Twisted Nematic) liquid crystal display element comprising a liquid crystal alignment direction of one side substrate placed at an angle greater than 180 degrees to a liquid crystal alignment direction of the other side substrate; (3) a TFT (Thin Film Transistor) liquid crystal display element which uses a thin film transistor.
However, the liquid crystal alignment films of the prior art have inferior rubbing resistance, which results in poor pretilt angle stability. Hence, in order to improve the resistance of alignment films; an epoxy compounds are added to the alignment agent, such as Japanese Patent Publication No. 07-234,410 which discloses using a compounds containing more than 2 epoxy function groups to serve as an additive agent to the liquid crystal alignment agent. However, such epoxy compounds in the alignment film results in an unreacted epoxy compounds after post-bake easily, causing image sticking problems.